Marantz 1060 (restored) - symetrical clipping won't adjust

david winter

Classical music lover.
Hi,

I'm just finishing a Marantz 1060 (I have done several so far without trouble).
In short, it's recaped with audio grade caps, the 2SC458 are replaced by KSC1845 as usual, etc.

My problem is that the lower part of the sine wave still shows a slipping whatever the adjustments.
Note that both bias set very nicely.

I replaced both 2SD438 (H703 / H704) which had a gain of about 55 (specs say 75min) and very oxidized legs. 2SC960 are shown in my service manual (I guess at least two referneces were used). Both original 2SD438 were replaced by NOS 2SD438 that I had in stock and which worked without trouble on a 1070. However, they have a gain of 120 to 160.
All other transistors on the output board are originals.

Is the problem due to the gain of those 2SD438 ? If so, is there a mod to accomodate this or should I simply use another type ? I'm asking because right now I don't have any 2N3440 (as suggested in other threads) but wonder if any other classic type could replace them.


Thanks !
 
I don't know if the substitute transistors are a problem or not as I have no experience with them.

The 'clipping' adjustment on this, the 1030 and the 2230 is kind of difficult to perform, as there is no 'hard' clip point.

What I find seems to work best is:
Working on one amp at a time, adjust the bias to spec, then advance the volume until you see a slight deformation on the top or bottom (positive or negative) of the sine wave looking across an 8Ω resistive load with your scope.
Adjust the clip point slightly, then go back and re-set the bias to spec. Repeat until you are satisfied that the deformation on the top and bottom of the output wave form occurs at as close to the same volume level as possible.
(you will have to run the volume up and down to look for this)
Once you have that, then run the volume down and up until you 'see' a slight increase in the THD+N of that loaded amp. Slightly 'tweak' the clipping adjustment for minimum distortion, then go back, re-set the bias and repeat this step ad nauseam.
Then do the other channel. The idea is to have the correct bias (volume all the way down) and the lowest THN+N at the highest loaded output level.

YMMV
Tom
 
I don't know if the substitute transistors are a problem or not as I have no experience with them.

The 'clipping' adjustment on this, the 1030 and the 2230 is kind of difficult to perform, as there is no 'hard' clip point.

What I find seems to work best is:
Working on one amp at a time, adjust the bias to spec, then advance the volume until you see a slight deformation on the top or bottom (positive or negative) of the sine wave looking across an 8Ω resistive load with your scope.
Adjust the clip point slightly, then go back and re-set the bias to spec. Repeat until you are satisfied that the deformation on the top and bottom of the output wave form occurs at as close to the same volume level as possible.
(you will have to run the volume up and down to look for this)
Once you have that, then run the volume down and up until you 'see' a slight increase in the THD+N of that loaded amp. Slightly 'tweak' the clipping adjustment for minimum distortion, then go back, re-set the bias and repeat this step ad nauseam.
Then do the other channel. The idea is to have the correct bias (volume all the way down) and the lowest THN+N at the highest loaded output level.

YMMV
Tom
I always wondered how it was done at the factory. Did they really sit and take all that time going back and forth between bias and clipping adjustments? Or did they just set the bias and set the clipping pot to some average voltage reading? With cap coupling the clipping seems to come on softer in the begining more like a tube amp. I adjusted clipping measuring across the coupling caps. A reading of half the voltage of the main filter cap gets you real close. After using a scope I didn't really hear a difference as to how it sounded or when it started clipping.
 
I may have been mistaken about which board the OP was referring too. My post was aimed at the power amp 'DC Balance' adjustment.
In my defense, Marantz called the phono board P700 for some models, P400 for others.

The Phono board "Symmetry' adjustment is even harder to set. The service documents call for setting it for equal clipping on both the positive and negative peaks of a 1kHz sine wave applied to the Phono In jacks, while observing the signal on an oscilloscope at the tape out jacks.
I have found that setting the adjustment pots for approximately 19VDC at the collectors of the final transistor stages is easier and gives good results.

Tom
 
I always wondered how it was done at the factory. Did they really sit and take all that time going back and forth between bias and clipping adjustments? Or did they just set the bias and set the clipping pot to some average voltage reading? With cap coupling the clipping seems to come on softer in the begining more like a tube amp. I adjusted clipping measuring across the coupling caps. A reading of half the voltage of the main filter cap gets you real close. After using a scope I didn't really hear a difference as to how it sounded or when it started clipping.

Catrafter's suggested procedures are right on and they're not that complicated, actually. As Field Engineer for HP for many years, some of the calibration procedures required that same back-and-forth motion of stepping through the same procedures until the results were optimized.

Some of these were quite tedious and required some special probes and test jigs for some of the tape drives, printers and disk drives. Believe me, when you had a disk drive the size of a washing machine (yep, old school!) and you were aligning the servo system that controlled the stepping of the heads, you had to get it right the first time.

Sorry to go off topic!

Cheers,

David
 
Hi,

I'm just finishing a Marantz 1060 (I have done several so far without trouble).
In short, it's recaped with audio grade caps, the 2SC458 are replaced by KSC1845 as usual, etc.

My problem is that the lower part of the sine wave still shows a slipping whatever the adjustments.
Note that both bias set very nicely.

I replaced both 2SD438 (H703 / H704) which had a gain of about 55 (specs say 75min) and very oxidized legs. 2SC960 are shown in my service manual (I guess at least two referneces were used). Both original 2SD438 were replaced by NOS 2SD438 that I had in stock and which worked without trouble on a 1070. However, they have a gain of 120 to 160.
All other transistors on the output board are originals.

Is the problem due to the gain of those 2SD438 ? If so, is there a mod to accomodate this or should I simply use another type ? I'm asking because right now I don't have any 2N3440 (as suggested in other threads) but wonder if any other classic type could replace them.


Thanks !

So, negative peak of the sine wave shows clipping before the positive? Or it clips at any output level? That would be a problem.
If it only clips at high levels, can you see that clipping responds to adjustment at all? If it does, set it to clip at highest output point, back down on signal input and measure your output voltage and convert into watts to see how far you are off spec 30wpc.
Uneven clipping effects max power output but not the sound quality as @catrafter explained to me when I had troubles adjusting mine.
I hope this helps!
 
First of all, thanks for your advises.
I realize that the clipping happens at the preamp output, so not within the output stage, which explains why I just cannot adjust it.
The question is, how can you adjust the clipping in the preamp area (or even the phono stage) ? There's no trimmer there and the entire preamp section isn't symetrical.
So I wonder what can cause this (and at the same time, whether the clipping adjustment really made sense).
Note that my amp is only recaped and all 2SC458 are replaced by KSC1845. Nothing else has been changed in the preamp section (except the 2SC458 in the phono stage).
Any advise appreciated (as always :) ).
 
What kind of output (in terms of wattage) do you get when the preamp is driven into clipping? It seems to me that you would be driving it very hard by then.

If that's the case, why worry about asymetrical clipping of the preamp when you'll never approach it with the power amp hooked up. Just set up the Power Amp as called for in the manual.

Now, if there's power supply problems in the preamp, this could manifest itself as asymetrical clipping there. Best to be sure all the operating voltages on the preamp stages are in spec first.

Cheers,

David
 
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